Reuters is reporting that a "small number" of Russian naval infrantry forces have joined the fight in Syria, alarming US officials who fear a Russian military escalation that could put Moscow on a collision course with Washington.

The Russians are reportedly constructing a large military facility in Latakia that could be used as an air base for operations against Islamic State and possibly US-backed rebels. This would complicate the US role in the conflict enormously.

The sources, speaking to Reuters on condition they not be identified, gave the most forthright account yet from the region of what the United States fears is a deepening Russian military role in Syria's civil war, though one of the Lebanese sources said the number of Russians involved so far was small.

U.S. officials said Russia sent two tank landing ships and additional cargo aircraft to Syriain the past day or so and deployed a small number of naval infantry forces.

The U.S. officials, who also spoke on condition of anonymity, said the intent of Russia's military moves in Syria was unclear. One suggested the focus may be on preparing an airfield near the port city of Latakia, a stronghold of Syrian President Bashar al-Assad.

U.S. officials have not ruled out the possibility that Russia may want to use the airfield for air combat missions.

U.S. Secretary of State John Kerry spoke to his Russian counterpart for the second time in four days to express concern over reports of Russian military activities in Syria, warning that it could fan more violence.

The White House said it was closely monitoring the situation.

Russia says the Syrian government must be incorporated into a shared global fight against Islamic State, the Islamist group that has taken over large parts of Syria and Iraq. The United States and Assad's regional foes see him as part of the problem.

"We would welcome constructive Russian contributions to the counter-ISIL effort, but we've been clear that it would be unconscionable for any party, including the Russians, to provide any support to the Assad regime," White House spokesman Eric Schultz said, using an acronym for Islamic State.

Assad's forces have faced big setbacks on the battlefield in a four-year-old multi-sided civil war that has killed 250,000 people and driven half of Syria's 23 million people from their homes.

Syrian troops pulled out of a major air base last Wednesday, and a monitoring group said this meant government soldiers were no longer present at all in Idlib province, most of which slipped from government control earlier this year.

President Assad is losing the war - slowly, painfully, but he is losing nonetheless. Now, his major benefactor may be about to enter the war, with uncertain consequences. The Russians have been pushing for a peace conference between Assad and the non-ISIS rebels for years, but the rebels insist any negotiations begin with President Assad stepping down. As long as Russia has anything to say about it, that's not going to happen but a de facto partition of Syria with an Alawite state in the west  protected by Moscow is not impossible to imagine.

The bottom line is that Russia will not allow Bashar Assad to fall. And Iran's Hezb'allah proxy army will help insure that won't happen.